Pulse modulated audio amplifiers are based on a switching principle which minimizes the power losses. The output transistor is only on or off—and does not work resistively like in traditional class amplifiers. Such switching causes high frequency energy and electromagnetic compatibility (EMC) problems to the output signals. There is high frequency energy because of a modulated square wave with a high carrier frequency. The high frequency energy is primarily in the carrier frequency and its harmonics.
Low pass filtering can be used to prevent unwanted energy going into the speaker which may damage it. The size of filtering can be minimized by using ternary modulation e.g. pseudo-differential or class-BD modulation where all idle switching is of the common mode type. High frequency energy causes EMC problems and the common mode noise is especially difficult to filter out. Common mode noise is a problem for example with a headset wire between the device and the earphones, wherein the headset wire acts as an antenna. This common mode noise can be filtered with relatively large filters (coils). The difficulties in the filtering have limited the commercial use in small and lightweight mobile electronic devices.